In the security field, duel technology motion detectors consisting of a Doppler microwave frequency (MW) detector and a passive infrared (PIR) detector are used to more accurately determine if an intruder has entered a protected space. The PIR detector senses infrared radiation (IR) from the intruder while the MW detector transmits a MW signal and receives a reflected MW signal from the intruder in the field. The typical operation of a dual technology motion detector is that the sensor will generate an alarm signal when the PIR detector and the MW detector both detect the intruder. The combination of the detectors eliminates the false alarm such as wall vibration while increasing the reliability. However, the combination of the detectors does not improve detection of an intruder when the intruder is cloaked, moves down the throat (directly at the detector), or is in a high ambient temperature. These situations are inherent weaknesses of the PIR detector.
It is desirable to produce a dual technology detector without the inherent weaknesses of the PIR detector and without the false alarms of the MW detector. This may be accomplished by using a reliable MW detector to generate an alarm signal without confirmation by the PIR detector. One way to achieve a reliable MW detector is to accurately determine the distance and movement of the target. This allows the detector to discriminate small moving objects close to the sensors that are too small to be intruders while reliably detecting an intruder at the back of the room. An accurate distance measurement also allows movement outside the protected space to be ignored and vibration of objects within the protected space to be discriminated against, thereby eliminating the MW detector's tendency to false alarm.
In addition, it is desirable to have a motion detector with greater pet immunity. When a household contains a pet, the PIR sensor's field of view is limited so it does not include the area near the floor, and therefore does not sense IR from a pet. However, a pet may trigger a false alarm when it is moving at the back of the room because the PIR sensor's field of view is cone shaped and at further distances senses IR closer to the floor. Accurate distance information would help distinguish this situation.
Unfortunately, the common Doppler MW motion sensors don't have the ability to measure target distance. A less expensive method of transmitting two different frequency MW signals and determining the phase difference between the two detected Doppler signals may be used to determine the distance, however, an ambiguity arises from a phase advance/delay uncertainty between the two signals.
It is object of the present invention to increase a dual technology detector's ability to detect an intruder.
It is a further object of the present invention to have a dual technology detector without the inherent weaknesses of the PIR motion detector.
It is a further object of the present invention to produce a MW motion detector that is not sensitive to false alarms.
It is a further object of the present invention to have a MW motion detector that accurately determines the distance of an object without ambiguity.
Finally, it is a further object of the present invention to have a detector with greater pet immunity.